We propose continued research into the structure, stability, and molecular interactions of selected human plasma proteins. These include (1) fibronectin, a multidomain glycoprotein which interacts with numerous macromolecules and mediates the attachment of various cells to surfaces, (2) complement C1, a large multisubunit assembly whose activation triggers an elaborate host-defense reaction, (3) C1-Inhibitor, a protease inhibitor which regulates the levels of active C1, and (4) antithrombin III, another inhibitor of several proteases in the blood clotting cascade. Spectroscopic techniques, especially fluorescence, will be used to investigate the kinetic and/or equilibrium aspects of functionally important protein-protein interactions such as fibronectin/collagen, fibronectin/heparin, C1-Inhibitor/C1 and between the various subcomponents of Cl (Clq, Clr, and Cls). These same techniques will be used to detect and characterize structural transitions of these proteins in response to various forms of stress (heat, denaturants, pH). Efforts will be made to identify independently unfolding domains and, where feasible, isolate them from enzymatic digests of the parent protein and further characterize them with respect to stability and interactions. Differential scanning calorimetry will be used in studies of thermal denaturation and results will be correlated with spectral measurements made under identical conditions. We will continue to seek conditions under which therapeutic plasma proteins can withstand pasteurization or other treatment, designed to selectively inactivate potentially infectious agents (hepatitis viruses, HTLV III) which might contaminate clinical preparations. In addition to a better understanding of structure and function, our studies may lead to the identification of new interactions, to a better understanding of the role of these proteins in pathological conditions and to the improved safety of transfusion products derived from human plasma.